A core in a boiling water reactor comprises a plurality of vertically arranged fuel assemblies. A fuel assembly comprises a bundle of vertical elongated fuel rods which are retained and fixed by a number of spacers arranged in spaced relationship to each other along the bundle. The spacers comprise a number of cells for mutually fixing the fuel rods. The ends of the bundle are retained at the bottom and at the top, by a bottom tie plate and a top tie plate. In certain fuel assemblies, the bundle is divided into four orthogonal sub-bundles and the lower and upper parts, of each sub-bundle are retained by a bottom tie plate and a top tie plate, respectively. The bundle and the sub-bundles, are surrounded by a fuel channel which is normally designed with a square cross section. The fuel assembly comprises a vertical channel containing non-boiling water surrounded by a tubular casing. In the following, the channel including its casing will be referred to as a water channel. The water channel extends through the whole fuel assembly and has a cross section which may be circular or cruciform.
The fuel rods contain a stack of pellets of a nuclear fuel arranged in a cladding tube. During the burnup of the nuclear fuel, fission gases contained inside the fuel rod are released. To prevent the pressure on the cladding from becoming too great, an expansion space for the fission gases is needed, a so-called fission gas plenum. The fission gas plenum is normally arranged above the stack of fuel pellets.
The core is immersed into water which serves both as a coolant and as a neutron moderator. During operation, the water flows from below and upwardly through the fuel assembly, whereby part of the water is transformed into steam. The proportion of steam is highest in the upper part of the fuel assembly. In the following, coolant means the water and the steam which flow through the fuel assembly. When the coolant flows upwardly through the fuel assembly, it is important that it be subjected to as little pressure drop as possible. Because of the high proportion of steam, the pressure drop is higher in the upper part of the fuel assembly than in the lower part thereof. During operation, the pressure drop across a spacer in the upper part of the fuel assembly is about five times higher than the pressure drop across a corresponding spacer in the lower part of the fuel assembly. Therefore, it is of particular importance to design the upper part of the fuel assembly in such a way that there is as low a pressure drop as possible.
A low pressure drop in the upper part of the fuel assembly is favorable for the stability properties of the fuel assembly and reduces the risk of dryout.
A high pressure drop means a high pressure on the fuel channel and may give rise to creeping and subsequently unacceptable deformation of the fuel channel.